1. Field of the Invention
About 11 years ago, i.e., TVA 10th Demonstration infra, production technology was developed to produce UAN suspensions with attapulgite clay and the chemical dispersant tetrasodium pyrophosphate (TSPP). About 7 years ago, i.e., TVA 12th Demonstration infra, the process was improved to produce such UAN attapulgite clay suspensions containing TSPP. Work in recent years has shown that sodium acid pyrophosphate (SAPP) could be used to chemically disperse sodium bentonite clay, however, there is an acute economic problem with both of these chemical dispersants in that they cost ten to twenty times as much as the eutectic UAN solution in which they are used. These same dispersants also cost ten to twenty times as much as the attapulgite or sodium bentonite clay. The new technology discussed herein presents an economically advantageous production scheme which mixes dry sodium bentonite clay directly in hot concentrated urea solution. The hot concentrated ammonium nitrate solution can then be added to this urea-clay-water mixture. This new technology is readily adaptable to a nitrogen fluid fertilizer production facility which has these hot concentrated streams of urea and ammonium nitrate. This new simplified technology will greatly reduce the investment cost and operating cost of the processing equipment.
For purposes of this disclosure the term UAN suspensions will be used even though at normally experienced ambient temperatures there are really no suspended crystals. The fluid fertilizer being discussed herein is actually a eutectic solution of urea, ammonium nitrate, and water in the presence of a sodium bentonite clay gel. Since the term UAN suspension is used within the fertilizer industry, the term will continue to be used in this disclosure. For more detailed information on the solubility isotherms and phase boundaries of the ammonium nitrate urea water system, as well as other properties thereof, see any number of references, including Agricultural Anhydrous Ammonia Technology and Use, Proceedings of the Agricultural Ammonia Institute, St. Louis, Mo., Sept. 29 and 30, 1965, of the Agricultural Ammonia Institute, Memphis, Tenn., Soil Science Society of America, Madison, Wis., Library of Congress Catalog Card No. 66-25830, specifically, but not necessarily exclusively, pages 46-48.
2. Description of the Prior Art
In U.S. Defensive Publication T911,008, June 5, 1973, Getsinger discusses the gelling of clay directly into UAN solution. The described process produces improved fluid nitrogen fertilizers by the addition of a gelling type clay and utilizing agitation for dispersion of the clay. In this work mechanical agitation is relied upon to disperse and gel the clay in a UAN solution.
In 1974 TVA researchers disclosed infra a method for producing a mixture of water, attapulgite clay, and tetrasodium pyrophosphate which could be added to urea ammonium nitrate solution. In this process a thirty weight percent clay dispersion was added to 34.3 percent nitrogen UAN solution to produce 32-0-0-2 clay. In 1978 a hot processing continuous production scheme was demonstrated by TVA researchers infra.
In this continuous production of UAN suspension, TSPP solution is prepared batchwise by mixing bagged TSPP with water. This 20 percent TSPP solution is fed simultaneously with 75 percent urea solution and water to a dilution funnel. Urea is introduced into and used in the dispersion step to avoid excessive dilution of the product. Temperatures of the TSPP solution and the urea solution are 160.degree. F. and 194.degree. F. (71.degree. C. and 90.degree. C.), respectively. Sufficient water is added in the funnel to reduce the concentration of urea solution to 50 percent urea. This mixture then flows into the clay dispersion tank where attapulgite clay is added. After a retention time of about 8 minutes, material overflows through a trough and is mixed with UAN solution as it enters the clay gelling tank. The clay gelling tank is baffled in such a manner that essentially all of the material passes through the recirculation pump.